An atropo-stereogenic diphosphane ligand with a proximal cationic charge: Specific catalytic properties of a palladium complex thereof

A class of cationic diphosphane ligands combining phosphane and amidiniophosphane moieties is illustrated on the N-methyl,N-naphthylbenzimidazolium framework. The palladium(II) complex thereof is described and compared to the corresponding complex of the analogous neutral diphos-. phane. Contrary to first-level expectations, the N2C-P and N2CP-Pd bonds in the cationic diphosphane complex are not longer than those occurring in its neutral counterpart. In the cationic ligand, the proximal positive charge is indeed conjugated to one phosphanyl group, and the coordination scheme is tentatively interpreted by resonance of the phosphane -> metal dative bond (+N2C-P:->[Pd+]) with a carbene -> phosphenium dative bond (N2C:->[P+:-> Pd]). Despite this peculiar structural feature, the electronic sigma donation (vs. pi acceptation) towards the palladium centre remains lowered in the cationic ligand. This specific property can be a priori valuable in a catalytic process where oxidative addition is not the limiting step. It is indeed shown that although the neutral complex is more active in Suzuki coupling reactions, the cationic complex is more active in Sonogashira-type coupling reactions involving predissociated halide substrates, namely an acyl chloride. These likely atropo-chiral ligands deserve to be resolved for application in asymmetric catalysis. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).